Substrate contact opening

ABSTRACT

An under-bump metallization (UBM) structure for a substrate, such as an organic substrate, a ceramic substrate, a silicon or glass interposer, a high density interconnect, a printed circuit board, or the like, is provided. A buffer layer is formed over a contact pad on the substrate such that at least a portion of the contact pad is exposed. A conductor pad is formed within the opening and extends over at least a portion of the buffer layer. The conductor pad may have a uniform thickness and/or a non-planar surface. The substrate may be attached to another substrate and/or a die.

This application is a divisional application of and claims the benefitof U.S. patent application Ser. No. 12/784,298, filed May 20, 2010 andentitled “Substrate Contact Opening,” which application is herebyincorporated herein by reference.

BACKGROUND

Since the invention of the integrated circuit (IC), the semiconductorindustry has experienced continued rapid growth due to continuousimprovements in the integration density of various electronic components(i.e., transistors, diodes, resistors, capacitors, etc.). For the mostpart, this improvement in integration density has come from repeatedreductions in minimum feature size, which allows more components to beintegrated into a given area.

The past few decades have also seen many shifts in semiconductorpackaging that have impacted the entire semiconductor industry. Theintroduction of surface-mount technology (SMT) and ball grid array (BGA)packages were generally important steps for high-throughput assembly ofa wide variety of IC devices, while at the same time allowing forreduction of the pad pitch on the printed circuit board (PCB).Conventionally packaged ICs have a structure basically interconnected byfine gold wire between metal pads on the die and electrodes spreadingout of molded resin packages. On the other hand, some BGA packages relyon bumps of solder to provide an electrical connection between contactson the die and contacts on a packaging substrate, such as a siliconinterposer, an organic substrate, a ceramic substrate, or the like, andrely on balls of solder to make an electrical connection betweencontacts on the packaging substrate and a PCB. Similarly, some chip sizepackaging (CSP) packages rely on balls of solder to make an electricalconnection directly between contacts on the die and a PCB, anotherdie/wafer, or the like. These techniques may also be used tointerconnect multiple dies and/or wafers. The various layers making upthese interconnections typically have different coefficients of thermalexpansion (CTEs). As a result, a relatively large stress may beexhibited on the joint area, which often causes cracks to form.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of contact structures that may be usedto provide an electrical connection to a substrate in accordance with anembodiment;

FIGS. 2A and 2B are cross-sectional views of contact structures that maybe used to provide an electrical connection to a substrate in accordancewith another embodiment; and

FIG. 3 is a cross-sectional view of a connection between a die and aprinted circuit board in accordance with yet another embodiment.

DETAILED DESCRIPTION

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use theembodiments, and do not limit the scope of the disclosure.

Embodiments described herein relate to the use of under-bumpmetallization (UBM) for use with semiconductor devices. As will bediscussed below, embodiments are disclosed that utilize a UBM structureon a substrate for the purpose of attaching one substrate to anothersubstrate, wherein each substrate may be a packaging substrate,interposer, printed circuit board, or the like. Other embodiments may beused to electrically couple one or more dies to a substrate, which inturn may be electrically coupled to another substrate. Embodiments maybe used in, for example, wafer level chip scale packaging (WLCSP)processes as well. Throughout the various views and illustrativeembodiments, like reference numerals are used to designate likeelements.

FIG. 1 illustrates a cross section of a portion of three substratesinterconnected in accordance with an embodiment. A first substrate 100may represent one or more dies having electrical circuitry thereon. Thefirst substrate 100 may comprise any suitable substrate, such as bulksilicon, doped or undoped, or an active layer of asemiconductor-on-insulator (SOI) substrate, or the like. The electricalcircuitry included on the first substrate 100 may be any type ofcircuitry suitable for a particular application. For example, theelectrical circuitry may include various N-type metal-oxidesemiconductor (NMOS) and/or P-type metal-oxide semiconductor (PMOS)devices, such as transistors, capacitors, resistors, diodes,photo-diodes, fuses, and the like, interconnected to perform one or morefunctions. The functions may include memory structures, processingstructures, sensors, amplifiers, power distribution, input/outputcircuitry, or the like. One of ordinary skill in the art will appreciatethat the above examples are provided for illustrative purposes only tofurther explain applications of some illustrative embodiments and arenot meant to limit the disclosure in any manner. Other circuitry may beused as appropriate for a given application.

The first substrate 100 further includes external contacts 102 forproviding an external electrical connection to the electrical circuitryformed thereon. A first passivation layer 104 is patterned to provide anopening over the external contacts 102 and to protect the underlyinglayers from various environmental contaminants. The upper dielectriclayer of the first substrate 100 is subject to oxidation when exposed tothe environment, and as a result, the first passivation layer 104 isformed over the uppermost dielectric layer. Thereafter, first conductivepads 106 are formed and patterned over the first passivation layer 104,such that at least a portion of the external contacts 102 are exposed. Asecond passivation layer 105, such as a polyimide layer (or otherpolymer material), is formed over the first passivation layer 104.Thereafter, an under bump metallization (UBM) layer 107 layer is formedand patterned over the second passivation layer 105 to provide anelectrical connection to the external contacts 102. The UBM layer 107may be formed of any suitable conductive materials, such as copper,tungsten, aluminum, silver, combinations thereof, or the like.

One of ordinary skill in the art will appreciate that the firstsubstrate 100 may include many other features. For example, the firstsubstrate 100 may include various metallization layers/dielectriclayers, vias, contacts, through-substrate vias, passivation layers,liners, adhesion/barrier layers, redistribution layers, and/or the like.

Any suitable process may be used to form the structures discussed aboveand will not be discussed in greater detail herein. As one of ordinaryskill in the art will realize, the above description provides a generaldescription of the features of the embodiment and that numerous otherfeatures may be present. For example, other circuitry, liners, barrierlayers, under-bump metallization configurations, and the like, may bepresent. The above description is meant only to provide a context forembodiments discussed herein and is not meant to limit the disclosure orthe scope of any claims to those specific embodiments.

FIG. 1 also illustrates a second substrate 120 and a third substrate 150in accordance with an embodiment. The second substrate 120 may be, forexample, an organic substrate, a ceramic substrate, a silicon or glassinterposer, a high-density interconnect, or the like. In an embodiment,the third substrate 150 is a PCB, but may include other types ofsubstrates. In some embodiments, the second substrate 120 may includeelectrical elements, such as capacitors, resistors, signal distributioncircuitry, and/or the like. These circuit elements may be active,passive, or a combination of active and passive elements.

The second substrate 120 has a bump stress buffer layer 122 along a dieside 124 and a first ball stress buffer layer 126 along an external side128. On the die side 124, the bump stress buffer layer 122 comprisesbump contacts 130 with a bump buffer layer 132 formed thereover. Thebump buffer layer 132 is patterned to provide openings over the bumpcontacts 130, thereby exposing at least a portion of the bump contacts130. Bump conductor pads 134 are formed within the openings andoverlapping at least a portion of the upper surface of the bump bufferlayer 132.

Similarly, on the external side 128, the first ball stress buffer layer126 comprises first ball contacts 140 with a first ball buffer layer 142formed thereover. The second substrate 120 may have one or more vias,e.g., through-substrate vias (not shown), providing an electricalconnection between respective ones of the first ball contacts 140 andthe bump contacts 130. The first ball buffer layer 142 is patterned toprovide openings over the first ball contacts 140, thereby exposing atleast a portion of the first ball contacts 140. First ball conductorpads 144 are formed within the openings and overlapping at least aportion of the upper surface of the first ball buffer layer 142.

The third substrate 150 has a second ball stress buffer layer 160, whichcomprises a second ball contact 162 with a second ball buffer layer 164formed thereover. The second ball buffer layer 164 is patterned toprovide openings over the second ball contacts 162, thereby exposing atleast a portion of the second ball contacts 162. Second ball conductorpads 166 are formed within the openings and overlapping at least aportion of the upper surface of the second ball buffer layer 164.

The bump buffer layer 132, the first ball buffer layer 142, and thesecond ball buffer layer 164, collectively referred to as the bufferlayer, may be formed of a dielectric material, such as polymer, epoxy,polyimide, solder resist, photoresist, or the like, by any suitablemethod, such as spray coating, roller coating, printing, filmlamination, or the like. In an embodiment, the buffer layer has athickness of about 1 μm to about 30 μm. Thereafter, the buffer layer maybe patterned using photolithography techniques by depositing aphotoresist material, exposing the photoresist material to a patterndefining the openings, and developing the photoresist material to removeunwanted portions of the photoresist material, or by laser drilling toremove unwanted portions of the photoresist material. Once formed, thepatterned photoresist material acts as an etch mask to pattern thebuffer layer as illustrated in FIG. 1. It should be noted that thebuffer layer itself may be formed of a photoresist material, such as aphoto-sensitive polymer, in which case the etching step may be omitted,or the buffer layer itself may be formed by laser drilling.

As illustrated by the inset in FIG. 1, in an embodiment the opening inthe buffer layer has a width D and a height H such that the aspect ratiodefined by H/D has a range of about 0.1 to about 1. In anotherembodiment, the width D is about one-half of the height H. It should benoted that in an embodiment, the sidewalls of the opening may beslanted, and in this embodiment, the width D may be measured at themidpoint of the sidewalls of the opening.

The bump conductor pad 134, the first ball conductor pad 144, and thesecond ball conductor pad 166, collectively referred to as the conductorpad, may be formed by any suitable method, including physical vapordeposition (PVD), chemical vapor deposition (CVD), electrochemicaldeposition (ECD), molecular beam epitaxy (MBE), atomic layer deposition(ALD), electroplating, and the like. In an embodiment, the conductor padis formed by depositing a conformal seed layer, which is a thin layer ofa conductive material that aids in the formation of a thicker layerduring subsequent processing steps. The seed layer may be formed bydepositing a thin conductive layer, such as a thin layer of Ni, Au, Cu,Ti, Ta, TiN, TaN, other inert metals, combinations thereof, or the like,using CVD or PVD techniques. Thereafter, a patterned mask is formed andpatterned over the seed layer that defines the lateral boundaries of theconductive pads. The patterned mask may be, for example, a patternedphotoresist mask, hard mask, a combination thereof, or the like.

After the patterned mask is formed over the seed layer, anelectroplating process may be performed to form the conductive pads tothe desired thickness. The conductor pad may be a single layer, acomposite layer, a stacked layer, or the like of one or more materialssuch as Ta, Ti, Ni, Au, Cu, Al, Pd, other inert metals, or the like. Forexample, in an embodiment the conductor pad comprises a layer of Cuabout 5 μm in thickness and an overlying layer of Ni about 3 μm inthickness. In an embodiment, the conductor pad is formed of a material,e.g., Ni, Pd, Au, or the like that prevents or reduces the ability ofcopper diffusion into the conductive bump/ball. In an embodiment, theconductive pads have a non-planar surface and have a uniform thicknessbetween less than about 10 μm. Once formed, the patterned mask and theremaining portions of the seed layer may be removed. In embodiments inwhich the patterned mask is formed from photoresist materials, thephotoresist may be stripped by, for example, a chemical solution such asa mixture of ethyl lactate, anisole, methyl butyl acetate, amyl acetate,cresol novolak resin, and diazo photoactive compound (referred to asSPR9), or another stripping process. A cleaning process, such as a wetdip in a chemical solution of phosphoric acid (H₃PO₄) and hydrogenperoxide (H₂O₂), referred to as DPP, with 2% hydrofluoric (HF) acid, oranother cleaning process, may be performed to remove exposed portions ofthe seed layer and any contaminants from the surface of the bufferlayer. It should be noted that the above process may result in the bumpconductor pad 134, the first ball conductor pad 144, and the second ballconductor pad 166 having rounded edges.

Conductive bumps 170 are used to provide an electrical connectionbetween the first substrate 100 and the second substrate 120, andconductive balls 172 are used to provide an electrical connectionbetween the second substrate 120 and the third substrate 150. In anembodiment, the second substrate 120 provides a matching coefficient ofthermal expansion to the first substrate 100 in order to reduce thepotential solder failure between the first substrate 100 and the secondsubstrate 120 caused by thermal stresses, such as a silicon interposer.The second substrate 120 also provides adaptation between smallercontact pads with reduced pitch on the first substrate 100 and largercontact pads with increased pitch on the third substrate 150. To aid inthe adaptation process and to allow different pin configurations betweenthe first substrate 100 and the third substrate 150, the secondsubstrate may include one or more redistribution layers (RDLs) on one orboth of the die side 124 and the external side 128. The conductive bumps170 and the conductive balls 172 may be lead free bumps, eutectic bumps,Cu post, or the like, and may comprise a homogeneous material property.

Optionally, an underfill 174 may be provided between the first substrate100 and the second substrate 120 and between the second substrate 120and the third substrate 150 to provide more reliable electricalconnection and protect the solder bumps/balls connecting the firstsubstrate 100 to the second substrate 120 and connecting the secondsubstrate 120 to the third substrate 150, respectively, from externalcontaminations, as appreciated by those skilled in the art.

FIGS. 2A and 2B illustrate a bump connection and a ball connection,respectively, in accordance with other embodiments. The embodimentillustrated in FIG. 2A is similar to the embodiment illustrated in FIG.1, except that the embodiment illustrated in FIG. 2A has two bump bufferlayers (132 ₁, 132 ₂) and two bump conductor pads (134 ₁, 134 ₂), whichmay be formed in a similar manner using similar techniques and materialsas described above with reference to the bump buffer layer 132 and thebump conductor pad 134, respectively.

Similarly, the embodiment illustrated in FIG. 2B is similar to theembodiment illustrated in FIG. 1, except that the embodiment illustratedin FIG. 2B has two ball buffer layers (164 ₁, 164 ₂) and two ballconductor pads (166 ₁, 166 ₂), which may be formed in a similar mannerusing similar techniques and materials as described above with referenceto the ball buffer layer 164 and the ball conductor pad 166,respectively. It should be noted that substrate 200 of FIG. 2Brepresents either the second substrate 120 or the third substrate 150 ofFIG. 1, such that the second substrate 120 and/or the third substrate150 may have two ball buffer layers 164 and two ball conductor pads 166.

It should be appreciated that embodiments such as those illustratedherein allow for the conductive bumps and/or conductive balls to have ahomogeneous material property. In connections used in other devices, theconductive bumps and/or conductive balls were surrounded by multipletypes of materials, such as a solder resist and an underfill material.In embodiments such as those discussed herein, however, the conductivebumps and/or conductive balls are only surrounded by a single type ofmaterial, e.g., the underfill material. As such, there is a singlematerial property exerting stress on the conductive bumps and/orconductive balls. It has also been found that the thermal stress betweenthe conductor pads and the conductive bumps/balls is less than it wouldbe if the conductive bumps/balls interfaced directly with the solderresist as in other systems.

FIG. 3 illustrates another embodiment, which may be used, for example,in the context of WLCSP. In this embodiment, a WLCSP die 300 is coupleddirectly to a PCB 302 by a conductive ball 372. The WLCSP die 300 hasformed thereon a WLCSP ball buffer layer 332 patterned to exposeunderlying WLCSP ball contacts 330. A WLCSP ball conductor pad 334 isformed within the opening of the WLCSP ball buffer layer 332 and extendsover a surface of the WLCSP ball buffer layer 332. The WLCSP ballcontacts 330, the WLCSP ball buffer layer 332, and the WLCSP ballconductor pad 334 may be formed using similar processes and materials asthe bump contacts 130, the bump buffer layer 132, and the bump conductorpad 134, respectively.

After the WLCSP processing has been completed, the WLCSP die 300 may beconnected to the PCB 302 using, for example, conductive balls 372, whichmay be similar to the conductive balls 172 discussed above. An optionalunderfill 374, which may be similar to the underfill 174 discussedabove, is also shown.

It should be noted that in this embodiment, the WLCSP die 300 is the dieitself having the conductive ball 372 applied thereto in accordance withWLCSP processing techniques. As such, this embodiment utilizes theconductive bump/ball contact structure discussed above with reference toFIGS. 1-2 b on the die in a WLCSP scheme as well as on the PCB 302.While some devices have in the past used conductive bump/ball structuressimilar to those discussed above on the die itself, the presentembodiments, among other things, utilize the conductive bump/ballstructures in a WLCSP scheme.

It has been found that embodiments such as those disclosed above mayreduce the stress between the conductive bumps/balls and the secondsubstrate and the third substrate, but also that using structures suchas these on the second substrate or the third substrate may also reducethe stress in the first substrate itself, e.g., the silicon die. Forexample, simulation results indicate that an embodiment in which asilicon die is attached directly to a substrate, in which the substrateutilizes a conductor pad such as those disclosed above, may reduce thestress not only between the bump and the substrate, but also between thebump and the silicon die itself. In particular, a simulation comparing aconnection between a substrate and a die wherein the substrate does nothave a conductor pad structure as discussed above and a connectionbetween a substrate and a die in which the substrate does have aconductor pad structure as discussed above, the stress in the areas ofthe interface between the bump and the die may be reduced at least asmuch as 32%.

An under-bump metallization (UBM) structure for a substrate, such as anorganic substrate, a ceramic substrate, a silicon or glass interposer, ahigh density interconnect, a printed circuit board, or the like, isprovided. A buffer layer is formed over a contact pad on the substratesuch that at least a portion of the contact pad is exposed. A conductorpad is formed within an opening and extends over at least a portion ofthe buffer layer. The conductor pad may have a uniform thickness and/ora non-planar surface. The substrate may be attached to another substrateand/or a die.

In another embodiment, an additional buffer layer may be formed andpatterned over the conductor pad such that at least a portion of theconductor pad is exposed. Thereafter, an additional conductor pad may beformed overlying at least a portion of the additional buffer layer andin contact with the underlying conductor pad.

Other embodiments are disclosed.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method of forming a semiconductor device, themethod comprising: providing a first substrate having a first electricalcontact on a first side and a second electrical contact on a secondside; forming a first buffer layer overlying the first electricalcontact, the first buffer layer having a first opening over at least aportion of the first electrical contact; forming a first conductor pad,the first conductor pad extending over a top surface of the first bufferlayer, the first conductor pad having a uniform thickness; providing asecond substrate having a third electrical contact on a third side;forming a second buffer layer overlying the third electrical contact,the second buffer layer having a second opening over at least a portionof the third electrical contact; forming a second conductor pad, thesecond conductor pad extending over a top surface of the second bufferlayer, the second conductor pad having a non-planar upper surface; andattaching the first substrate to the second substrate using a conductiveelement, the conductive element being in direct contact with the firstconductor pad and the second conductor pad.
 2. The method of claim 1,wherein the first buffer layer comprises a polymer, an epoxy, apolyimide, a solder resist, or a photo-sensitive material.
 3. The methodof claim 1, wherein the first buffer layer comprises a bump bufferlayer, and the first conductor pad comprises a first bump conductor pad.4. The method of claim 1, wherein the first buffer layer comprises aball buffer layer, and the first conductor pad comprises a ballconductor pad.
 5. The method of claim 1, further comprising: forming athird buffer layer over the first conductor pad, the third buffer layerhaving a third opening over at least a portion of the first conductorpad; and forming a third conductor pad, the third conductor padextending over a top surface of the third buffer layer and having auniform thickness.
 6. The method of claim 1, wherein the first substratecomprises an organic substrate, a high density interconnect, a printedcircuit board, or a silicon interposer.
 7. The method of claim 1,wherein a thickness of the first conductor pad is less than about 10 μm.8. The method of claim 1, wherein the first conductor pad comprises acomposite layer.
 9. A method of forming a semiconductor device, themethod comprising: providing a first substrate having a first contactpad on a first side; forming one or more buffer layers overlying thefirst contact pad, wherein a first buffer layer of the one or morebuffer layers has an opening, at least a portion of the first contactpad being exposed in the opening; forming one or more conductor pads onthe first side of the first substrate, wherein a first conductor pad ofthe one or more conductor pads extends over sidewalls of the opening inthe first buffer layer, the first conductor pad having a non-planarupper surface and extending over at least a portion of the first bufferlayer; providing an integrated circuit having a second substrate, thesecond substrate having a second contact pad on a second side; forming apassivation layer overlying the second contact pad; forming a secondconductor pad on the second side of the second substrate, a firstportion of the second conductor pad extending through the passivationlayer and contacting the second contact pad, a second portion of thesecond conductor pad extending over a top surface of the passivationlayer; and forming a conductive element interposed between the firstsubstrate and the second substrate, the conductive element being indirect contact with an uppermost conductor pad of the first substrateand the second conductor pad of the second substrate; wherein each ofany remaining buffer layers has an opening exposing at least a portionof an underlying conductor pad; and wherein each of any remainingconductor pads extends over sidewalls of a corresponding one of thebuffer layers, each of the remaining conductor pads having a non-planarupper surface and extending over at least a portion of the correspondingone of the buffer layers.
 10. The method of claim 9, wherein theconductive element is a conductive bump element.
 11. The method of claim10, wherein the conductive bump element is a solder bump.
 12. The methodof claim 9, wherein the one or more conductor pads have a uniformthickness.
 13. The method of claim 9, wherein the semiconductor devicecomprises two buffer layers.
 14. The method of claim 9, wherein the oneor more buffer layers are one or more bump buffer layers and the one ormore conductor pads are one or more bump conductor pads, and furthercomprising: forming a ball buffer layer on a third side of the firstsubstrate, the third side of the first substrate being opposite thefirst side of the first substrate, the ball buffer layer having anopening over at least a portion of an electrical contact; and forming aball conductor pad, the ball conductor pad having a uniform thicknessand extending over a top surface of the ball buffer layer, the ballconductor pad contacting the electrical contact.
 15. The method of claim14, further comprising: providing a third substrate having anotherbuffer layer, wherein the another buffer layer has another conductorpad, the another conductor pad extending through the another bufferlayer and having a uniform thickness; and forming another conductiveelement interposed between the first substrate and the third substrate,the another conductive element being in direct contact with the ballconductor pad of the first substrate.
 16. A method of forming asemiconductor device, the method comprising: providing a first substratehaving a first electrical contact on a first side and a secondelectrical contact on a second side; forming a first bump buffer layeroverlying the first electrical contact, the first bump buffer layerhaving a first opening over at least a portion of the first electricalcontact; forming a first bump conductor pad, the first bump conductorpad extending over a top surface of the first bump buffer layer, thefirst bump conductor pad having a uniform thickness; forming a firstball buffer layer overlying the second electrical contact, the firstball buffer layer having a second opening over at least a portion of thesecond electrical contact; forming a first ball conductor pad, the firstball conductor pad extending over a top surface of the first ball bufferlayer, the first ball conductor pad having a uniform thickness; forminga solder bump, the solder bump being in direct contact with the firstbump conductor pad; forming a solder ball, the solder ball being indirect contact with the first ball conductor pad; attaching a secondsubstrate to the first side of the first substrate, an external contactpad of the second substrate being in direct contact with the solderbump; and attaching a third substrate to the second side of the firstsubstrate, an external contact pad of the third substrate being indirect contact with the solder ball.
 17. The method of claim 16, whereinthe first ball conductor pad has a non-planar upper surface.
 18. Themethod of claim 16, wherein the first bump conductor pad has anon-planar upper surface.
 19. The method of claim 16, furthercomprising: forming a second bump buffer layer under the first bumpbuffer layer, the second bump buffer layer having an opening over atleast a portion of the first electrical contact; and forming a secondbump conductor pad, the second bump conductor pad extending over a topsurface of the second bump buffer layer, the second bump conductor padhaving a uniform thickness.
 20. The method of claim 16, furthercomprising: forming a second ball buffer layer under the first ballbuffer layer, the second ball buffer layer having an opening over atleast a portion of the second electrical contact; and forming a secondball conductor pad, the second ball conductor pad extending over a topsurface of the second ball buffer layer, the second ball conductor padhaving a uniform thickness.